Sensitive Glycoprotein Sandwich Assays by the Synergistic Effect of In

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Sensitive Glycoprotein Sandwich Assays by the Synergistic Effect of In Situ Generation of Raman Probes and Plasmonic Coupling of Ag Core−Au Satellite Nanostructures Xiaoshuang Bi, Xueyuan Li, Dong Chen, and Xuezhong Du* Key Laboratory of Mesoscopic Chemistry (Ministry of Education), State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Chemistry for Life Sciences, and School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210023, People’s Republic of China S Supporting Information *

ABSTRACT: Sensitive surface-enhanced Raman scattering (SERS) assays of glycoproteins have been proposed using p-aminothiophenol (PATP)-embedded Ag core−Au satellite nanostructures modified with p-mercaptophenylboronic acid (PMBA) and the self-assembled monolayer of PMBA on a smooth gold-coated wafer. The apparent Raman probe PATP on the surfaces of the Ag cores underwent a photodimerization to generate 4,4′-dimercaptoazobenzene (DMAB) in situ upon excitation of laser, and the in situ generated DMAB acted as the actual Raman probe with considerably strong SERS signals, which was further enhanced by the plasmonic coupling of the Ag core−Au satellite nanostructures due to the synergistic effect. The sandwich assays of glycoproteins showed high sensitivity and excellent selectivity against nonglycoproteins. The Ag core−Au satellite SERS nanostructures can be used for highly sensitive SERS assays of other analytes. KEYWORDS: Ag core−Au satellite nanostructure, glycoprotein, plasmonic coupling, sandwich assay, SERS

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INTRODUCTION Glycoproteins consist of polypeptides covalently linked with carbohydrate moieties and play important roles in many biological processes, such as molecular recognition, cell signaling, immune response, and regulation of cellular development.1 Glycoproteins are related to the occurrence and development of a variety of diseases, such as infection, cancer, cardiovascular disease, liver disease, kidney disease, and diabetes, and thus, glycoproteins can be used as disease biomarkers for clinical diagnostics.2 Glycoproteins are found in relatively low abundance (2−5%) in typical glycoprotein mixtures derived from cells in comparison to nonglycoproteins.3 α-Fetoprotein (AFP) is an oncofetal glycoprotein and is widely used as a tumor biomarker, and the concentration of serum AFP in healthy adults is generally very low ( 99.8%) was purchased from Shanghai Shenbo Chemical Reagent Co., Ltd. (China). Trisodium citrate dehydrate (99%), hydroxylamine (NH2OH·HCl), anhydrous ethanol, hydrogen peroxide (30%), and concentrated sulfuric acid (98%) were purchased from Nanjing Chemical Reagent Co., Ltd. (China). Polyvinylpyrrolidone (PVP) was purchased from Sinopharm Chemical Reagent Co., Ltd., and PATP was from Adamas-beta. Tetrachloroauric acid (HAuCl4) and PMBA were obtained from Sigma-Aldrich. Horseradish peroxidase (HRP, 44 kDa, pI 7.2) was offered by Sangon Biotech Co., Ltd. (Shanghai), and bovine serum albumin (BSA, 67 kDa, pI 4.7) was by Amresco. The water used was double-distilled water. The aqueous protein solutions were prepared with phosphate-buffered solutions (PB, pH 7.4), and the solutions of PMBA and PTAP were prepared with anhydrous ethanol. 10684

DOI: 10.1021/acsami.6b00450 ACS Appl. Mater. Interfaces 2016, 8, 10683−10689

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ACS Applied Materials & Interfaces PATP and HAuCl4, under stirring for 10 min followed by aging without stirring for 12 h. Self-Assembly of PMBA on Gold-Coated Silicon Wafers. Clean single-crystal silicon (111) wafers were used to deposit the thin gold films of 100 nm thick using an electron beam evaporation system.25 The gold-coated silicon wafers were cut into the sizes of 1 × 1 cm and then placed into piranha solution (H2O2/H2SO4, 1:3, v/v) for 1 h, followed by rinsing with water and drying in the air. The cleaned gold-coated silicon wafers were placed into anhydrous ethanol containing PMBA (5 mmol/L) for 24 h to form self-assembled monolayers (SAMs). Afterward, the SAMs on the gold-coated silicon wafers were washed thoroughly with anhydrous ethanol and dried in the air. Sandwich Assays of Glycoproteins. The PMBA-functionalized gold-coated silicon wafers were placed into aqueous glycoprotein solutions of different concentrations followed by washing with water to remove uncaptured glycoproteins. Then, the glycoprotein-captured SAMs of PMBA were placed into the hydrosol of PMBA-modified SERS tags, and uncaptured PMBA-modified SERS tags were washed with water followed by drying in the air prior to SERS measurement. Similarly, the same procedures were used for the assays of nonglycoproteins for comparison. Instruments and Measurements. Transmission electron microscopy (TEM) images were obtained on a JEM-2100 microscope. UV−vis spectra were collected on a spectrophotometer (Model UV3600, Shimadzu) in quartz cells with an optical length of 0.3 cm. X-ray diffraction (XRD) patterns were acquired on an X-ray powder diffractometer (Philips PANAlytical X’pert) with Cu Kα radiation. Raman spectra were recorded on a LabRAM Aramis HJY Raman spectrometer with a CCD detector. The lasers with different excitation wavelengths (532, 633, and 785 nm) were used for spectral measurements. Before spectral measurements, the Si standard (Raman signal at 520 cm−1) was used for the calibration of the instrument. The laser power was set for 5 mW, and the laser beam was focused to a spot of 1 μm in diameter. The exposure time for spectral measurements was 10 s, and SERS spectra were collected by coaddition of 4 scans with the resolution of 4 cm−1. The resulting SERS spectra were acquired in two modes: (1) the SERS tags of Ag core−Au satellite/shell nanostructures modified with PMBA were measured in their hydrosols; (2) glycoproteins sandwiched between the SERS tags and the SAMs of PMBA were measured in the air. Three independent samples were prepared for the SERS measurements, and five replicates were measured from five random sites on the sample surface for each sample. The error bars were the standard deviation from a total of 15 tests. All of the SERS spectra shown were baseline-corrected but not normalized.

Figure 1. TEM images of AgNPs and Ag core−Au satellite/shell nanostructures, corresponding to different amounts of aqueous HAuCl4 solution (0.465 mmol/L) added (mL): (a) 0; (b) 1.0; (c) 2.0; (d) 3.0; (e) 4.0; and (f) 5.0.

gap of ca. 2 nm (Figure 1c), and finally with complete Au shells of different thicknesses (Figure 1d−f). Because Au (a = 0.40783 nm) and Ag (a = 0.40862 nm) have the similar lattice structures,24 it is difficult to distinguish Au from Ag using the XRD technique26 (Figure S1, Supporting Information). However, owing to the different atomic weights of Au and Ag (196.97 and 106.87, respectively), Au and Ag can be distinguished by high-resolution TEM (HRTEM) technique,26 although lattice spacings between neighboring fringes in Au and Ag are similar. The dark area corresponded to Au and the light area to Ag.27 The Ag core−Au satellite/shell nanostructures were formed with obvious boundaries from the HRTEM image (Figure S2, Supporting Information). Energy dispersive X-ray (EDX) analysis further confirmed the presence of Ag and Au elements in the Ag core−Au satellite/shell nanostructures (Figure S3, Supporting Information). The LSPR band of the as-synthesized AgNPs appeared at 403 nm, and the absorption maximum gradually shifted to 425 nm concomitant with the decreased intensity and the broadened band with the increase of the amount of HAuCl4 added, owing to the formation of AuNPs on the surfaces of the AgNP cores and the superposition of their LSPR bands (Figure 2). Upon further increase of HAuCl4 amount, an obvious band appeared in the range of 510−620 nm due to the LSPR absorption of AuNPs or Au shells, while the LSPR band of AgNPs was progressively weakened, which indicates that the Au shell gradually increased in thickness. The corresponding hydrosols underwent an obvious change in color from the yellowish green of AgNPs to the reddish brown, purple, and blue, which suggested the formation of the Ag core−Au satellite and Ag core−Au shell nanostructures in turn.

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RESULTS AND DISCUSSION Preparation of Ag Core−Au Satellite SERS Nanostructures. Citrate-coated AgNPs with the diameter of 60 ± 5 nm were first synthesized (Figure 1a) and then modified with PATP. The Ag core−Au satellite/shell nanostructures of 70−90 nm in diameter were prepared by addition of aqueous HAuCl4 solution with hydroxylamine as a reducing agent and PVP as a stabilizing agent (Figure 1b−f) using the modified seed-growth method of Srnova-Sloufova et al., who synthesized the Ag core−Au shell nanostructures of 10−16 nm in diameter without any stabilizing agent.24 Despite the common galvanic replacement reaction between the Ag cores and HAuCl4, both the protection of the modified PATP layer and PVP stabilizing agent and the use of the efficient reducing agent (hydroxylamine) for deposition of a second metal layer over nuclei24 could block the replacement reaction to some extent for the preparation of Ag core−Au satellite/shell nanostructures. With the increase of the amount of HAuCl4 added, AgNP cores were first coated with a few AuNPs (Figure 1b), then with a large number of AuNPs of 12−13 nm in diameter at the interparticle 10685

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increased in intensity owing to the electromagnetic field enhancement via plasmonic coupling for the generation of “hot spots” (curves b and c).30−32 Upon increase of the amount of AuNPs, the intensities of the SERS peaks reached maxima for the AgNP cores coated with a number of AuNPs at the small gap of ca. 2 nm (curve d), owing to the considerably strong plasmonic coupling not only between the AgNP cores and AuNP satellites but also between the AuNP satellites. The SERS peak intensities were 17 times stronger than those of the PATP-modified AgNPs in the absence of AuNPs (curve a). Upon increase of the amount of HAuCl4 added, the compactpacked AuNPs gradually developed a complete Au shell, whereas the SERS peak intensities progressively decreased (curves e and f), which means that the electromagnetic field was accordingly reduced in this case. A reduction in electromagnetic field was caused by few surface charges concentrating within the compact-packed AuNPs.33 The SERS peaks decreased in intensity upon further increase of Au shell thickness (curves g and h) due to the increase of optical path and the deviation of laser focus.34 It is clear that the Ag core−Au satellite nanostructures had much higher SERS activity due to the generation of lots of “hot spots” for the strong plasmonic coupling than the bare AgNPs and the Ag core−Au shell nanostructures. Owing to the synergistic effect of the large Raman scattering cross section of the in situ-generated Raman probes and the electromagnetic field enhancement of plasmonic coupling, the sensitivity of glycoprotein assays using the Ag core−Au satellite SERS nanostructures would be enormously improved in comparison to that using the AgNPs. Optimization of Parameters. The SERS activities of the Ag core−Au satellite/shell nanostructures were further compared at different excitation wavelengths (532, 633, and 785 nm; Figure S5, Supporting Information). The change trends of the SERS activities were similar at different excitation wavelengths, that is, the Ag core−Au satellite nanostructures with considerably enhanced electromagnetic field showed stronger SERS activities than either AgNPs or the Ag core− Au shell nanostructures. Moreover, the SERS activity of the Ag core−Au satellite nanostructures was highest at the excitation wavelength of 532 nm. It is known that the maximum SERS enhancement occurs when the wavelength of LSPR is located between the excitation wavelength and the wavelength of scattered Raman spectra of analytes.35 The LSPR wavelengths of the prepared Ag core−Au satellite/shell nanostructures were far away from the excitation wavelength of 785 nm. It is reported that the AuNP-immobilized silver film over nanospheres (AgFON) arrays showed the strongest SERS activity at the nanosphere size of 505 nm for all the excitation wavelengths (532, 633, and 785 nm),36 and the size and shape of AuNPs also affected the SERS activity of the substrates dependent on the surface density of AuNPs.36 The amount of PATP added was further optimized for the preparation of the Ag core−Au satellite SERS nanostructures (Figure S6, Supporting Information). When 75 μL of PATP in ethanol (0.1 mmol/L) was added (surface coverage of about 75% on the AgNP cores, Supporting Information), the intensity of the SERS peak at 1436 cm−1 reached a saturated value, and then the peak intensities remained almost unchanged upon further increase of the amount of PATP. Glycoprotein Sandwich Assays. The SAMs of PMBA were incubated with aqueous HRP solutions of different concentrations, followed by washing to remove uncaptured HRP. The carbohydrate moieties of HRP were captured by the

Figure 2. UV−vis absorption spectra of the hydrosols of AgNPs and Ag core−Au satellite/shell nanostructures, corresponding to different amounts of aqueous HAuCl4 solution (0.465 mmol/L) added (mL): (a) 0.0; (b) 1.0; (c) 1.5; (d) 2.0; (e) 2.5; (f) 3.0; (g) 4.0; and (h) 5.0. (Inset) The corresponding photographic images.

The PATP-modified AgNPs showed the main SERS peaks at 1579, 1436, 1390, 1140, and 1070 cm−1 (Figure 3, curve a).

Figure 3. SERS spectra of the hydrosols of PATP-embedded Ag core− Au satellite/shell nanostructures, corresponding to different amounts of aqueous HAuCl4 solution (0.465 mmol/L) added (mL): (a) 0.0; (b) 1.0; (c) 1.5; (d) 2.0; (e) 2.5; (f) 3.0; (g) 4.0; and (h) 5.0.

These SERS peaks did not originate from PATP itself28 but from its photodimerization product DMAB generated in situ on the AgNP surfaces under the excitation of laser.15−20 The Raman shifts at 1436 and 1390 cm−1 are owing to the NN stretching vibrations of DMAB.15,16,22 The true SERS shifts of PATP appeared at 1582 and 1078 cm−1 by means of alkyl layerisolated nanoparticle-enhanced Raman spectroscopy (ALINERS)29 (Figure S4, Supporting Information) and ultrathin silica shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)10,16,17,22 to separate PATP from direct contact with SERS-active substrates to prohibit its photodimerization transformation. The intensities of the SERS peaks of DMAB were much stronger than those of typical SERS peaks of PATP (1582 and 1078 cm−1)29 and PMBA (1570 and 1070 cm−1).25 The Raman scattering cross section of DMAB was reported to be more than 3 orders of magnitude greater than that of benzenethiol derivatives (including PATP and PMBA) with the synergistic effect of resonance Raman scattering and binding to AgNPs.21,22 When the AgNP cores were coated with AuNPs, the SERS peaks of the in situ-generated DMAB significantly 10686

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gold-coated silicon wafers used were easy to separate, detect, and reuse. The intensity of the peak at 1436 cm−1 as a function of HRP concentration is plotted in Figures 4b. In the wide range of HRP concentrations from 10 ng/mL to 1 mg/mL, there was an almost linear relationship in a bilogarithmic plot. Comparisons of the detection limit and concentration range of glycoproteins and pH of solutions in this work with those reported in the literature are listed in Table S1 in the Supporting Information. It is clear that our SERS sandwich assay showed a lower detection limit and a wider concentration range for the detection of glycoproteins. Selectivity of Glycoprotein Assays. To demonstrate the selectivity of glycoprotein sandwich assays, BSA (nonglycoprotein) was tested using the SAM of PMBA and the Ag core−Au satellite SERS nanostructures modified with PMBA (Figure 5).

boronic acid of PMBA in the SAM through a cyclic boronate linkage. Afterward, the HRP-captured SAMs of PMBA were further incubated with the hydrosol of the Ag core−Au satellite SERS nanostructures modified with PMBA, followed by washing to remove uncaptured SERS tags. As a result, the SERS tags were captured by the SAMs of PMBA via the glycoprotein sandwich structure with dual cyclic boronate linkages. In the absence of HRP, no Raman signal could be detectable. This is because the SAM of PMBA on a smooth gold-coated silicon wafer itself could not yield Raman signal and the PMBA-modified SERS tags could not be captured by the SAM of PMBA in the absence of HRP. In the presence of HRP, the SERS peaks of the in situ-generated DMAB from the captured SERS tags could be observed and increased in intensity with the increase of HRP concentration (Figure 4a).

Figure 4. (a) SERS spectra of the PMBA-modified gold-coated silicon slides incubated with HRP of different concentrations followed by incubating with the Ag core−Au satellite SERS nanostructures, washing, and drying. (b) Intensity of the SERS peak at 1436 cm−1 as a function of concentration of HRP. Inset shows an almost linear relationship in a bilogarithmic plot.

Figure 5. (A) SERS spectra of the PMBA-modified gold-coated silicon wafers incubated with aqueous protein solutions: (a) BSA (0.5 mg/ mL); (b) HRP (0.5 mg/mL); (c) mixture of HRP (0.5 mg/mL) and BSA (0.5 mg/mL), followed by incubating with the Ag core−Au satellite SERS nanostructures, washing, and drying. (B) Histogram indicates the SERS intensities at 1436 cm−1 detected in the presence of proteins.

Even at the concentration of HRP as low as 0.00001 mg/mL (10 ng/mL), the SERS peaks were still could be detected. It is further confirmed that the glycoproteins were assayed with the sandwich structure via the formation of dual boronate linkages (Figure S7, Supporting Information). The assays of glycoproteins were performed using the smooth gold-coated silicon wafers in the neutral PB solutions (pH 7.4). In comparison with the assays of glycoproteins using nanoparticle powders in alkaline solutions (preferential formation of boronate bonds),37 the measured detection limit (10 ng/mL) in our case was very low, which is able to directly determine glycoproteins without the need of enrichment using the SERS sandwich assay. The

However, no SERS signal could be detectable at the concentration as high as 0.5 mg/mL because BSA contains no carbohydrate moiety and could not be captured for the formation of the sandwich structure. In the presence of HRP and BSA at the same concentration, the intensities of the SERS peaks were almost identical to those in the presence of HRP only. Serum albumin is the most abundant protein in plasma and accounts for 60% of total plasma proteins. These comparative studies confirm the selectivity of glycoprotein sandwich assays. It is obvious that the developed SERS 10687

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(8) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667− 1670. (9) Camden, J. P.; Dieringer, J. A.; Wang, Y.; Masiello, D. J.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P. Probing the Structure of SingleMolecule Surface-Enhanced Raman Scattering Hot Spots. J. Am. Chem. Soc. 2008, 130, 12616−12617. (10) Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; Wu, D. Y.; Ren, B.; Wang, Z. L.; Tian, Z. Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010, 464, 392−395. (11) Xu, W.; Ling, X.; Xiao, J.; Dresselhaus, M. S.; Kong, J.; Xu, H.; Liu, Z.; Zhang, J. Surface Enhanced Raman Spectroscopy on a Flat Graphene Surface. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 9281− 9286. (12) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Saccharide Sensing with Molecular Receptors Based on Boronic Acid. Angew. Chem., Int. Ed. 1996, 35, 1910−1922. (13) Yang, W.; He, H.; Drueckhammer, D. G. Computer-Guided Design in Molecular Recognition: Design and Synthesis of a Glucopyranose Receptor. Angew. Chem., Int. Ed. 2001, 40, 1714−1718. (14) Kong, K. V.; Lam, Z.; Lau, W. K. O.; Leong, W. K.; Olivo, M. A Transition Metal Carbonyl Probe for Use in a Highly Specific and Sensitive SERS-Based Assay for Glucose. J. Am. Chem. Soc. 2013, 135, 18028−18031. (15) Fang, Y.; Li, Y.; Xu, H.; Sun, M. Ascertaining p,p′Dimercaptoazobenzene Produced from p-Aminothiophenol by Selective Catalytic Coupling Reaction on Silver Nanoparticles. Langmuir 2010, 26, 7737−7746. (16) Huang, Y.-F.; Zhu, H.-P.; Liu, G.-K.; Wu, D.-Y.; Ren, B.; Tian, Z.-Q. When the Signal Is Not from the Original Molecule To Be Detected: Chemical Transformation of para-Aminothiophenol on Ag during the SERS Measurement. J. Am. Chem. Soc. 2010, 132, 9244− 9246. (17) Huang, Y.-F.; Wu, D.-Y.; Zhu, H.-P.; Zhao, L.-B.; Liu, G.-K.; Ren, B.; Tian, Z.-Q. Surface-Enhanced Raman Spectroscopic Study of p-Aminothiophenol. Phys. Chem. Chem. Phys. 2012, 14, 8485−8497. (18) Sun, M.; Xu, H. A Novel Application of Plasmonics: PlasmonDriven Surface-Catalyzed Reactions. Small 2012, 8, 2777−2786. (19) Choi, H.-K.; Shon, H. K.; Yu, H.; Lee, T. G.; Kim, Z. H. b2 Peaks in SERS Spectra of 4-Aminobenzenethiol: A Photochemical Artifact or a Real Chemical Enhancement? J. Phys. Chem. Lett. 2013, 4, 1079−1084. (20) Zhang, Z.; Fang, Y.; Wang, W.; Chen, L.; Sun, M. Propagating Surface Plasmon Polaritons: Towards Applications for RemoteExcitation Surface Catalytic Reactions. Adv. Sci. 2016, 3, 1500215. (21) Wu, D.-Y.; Zhao, L.-B.; Liu, X.-M.; Huang, R.; Huang, Y.-F.; Ren, B.; Tian, Z.-Q. Photon-Driven Charge Transfer and Photocatalysis of p-Aminothiophenol in Metal Nanogaps: A DFT Study of SERS. Chem. Commun. 2011, 47, 2520−2522. (22) Wu, D.-Y.; Liu, X.-M.; Huang, Y.-F.; Ren, B.; Xu, X.; Tian, Z.-Q. Surface Catalytic Coupling Reaction of p-Mercaptoaniline Linking to Silver Nanostructures Responsible for Abnormal SERS Enhancement: a DFT Study. J. Phys. Chem. C 2009, 113, 18212−18222. (23) Lee, P. C.; Meisel, D. Adsorption and Surface-Enhanced Raman of Dyes on Silver and Gold Sols. J. Phys. Chem. 1982, 86, 3391−3395. (24) Srnova-Sloufova, I.; Lednicky, F.; Gemperle, A.; Gemperlova, J. Core-Shell (Ag)Au Bimetallic Nanoparticles: Analysis of Transmission Electron Microscopy Images. Langmuir 2000, 16, 9928−9935. (25) Bi, X.; Du, X.; Jiang, J.; Huang, X. Facile and Sensitive Glucose Sandwich Assay using in Situ-Generated Raman Reporters. Anal. Chem. 2015, 87, 2016−2021. (26) Fu, H.; Yang, X.; Jiang, X.; Yu, A. Bimetallic Ag−Au Nanowires: Synthesis, Growth Mechanism, and Catalytic Properties. Langmuir 2013, 29, 7134−7142. (27) Seo, D.; Yoo, C. I.; Jung, J.; Song, H. Ag−Au−Ag Heterometallic Nanorods Formed through Directed Anisotropic Growth. J. Am. Chem. Soc. 2008, 130, 2940−2941.

sandwich assay may be used for the detection of glycoproteins in biological samples.

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CONCLUSIONS The sensitive SERS assays of glycoproteins were performed in combination of the PATP-embedded Ag core−Au satellite SERS nanostructures modified with PMBA and the SAMs of PMBA. The in situ-generated DMBA, from the photodimerization of PATP on the surfaces of the Ag cores under the excitation of laser, acted as the actual Raman probe and had considerably strong characteristic SERS signals, which was further enhanced by the plasmonic coupling of the Ag core−Au satellite nanostructures due to the synergistic effect. The sandwich assays of glycoproteins showed high sensitivity and excellent selectivity against nonglycoproteins. The Ag core−Au satellite SERS nanostructures can be used for the highly sensitive SERS assays of other analytes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00450. XRD patterns, HRTEM image, and EDX analysis of the Ag core−Au satellite/shell nanostructures, true SERS spectrum of PATP, SERS activities of the Ag core−Au satellite/shell nanostructures at different excitation wavelengths and different PATP amounts, SERS spectra of PMBA-modified smooth gold-coated silicon wafer and PMBA-modified Ag core−Au satellite nanostructures and SERS tags, and comparisons of the detection of a few types of glycoproteins in the literature. (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Fax: 86-25-89687761. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21273112). REFERENCES

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